Fall Armyworm

Spodoptera frugiperda (J.E. Smith)

There are two host strains of fall armyworms: the rice strain and the corn strain. The rice strain is associated with rice, Bermudagrass, and other pasture grasses. The corn strain is the predominant fall armyworm on corn, sorghum, and cotton. Other than feeding habits, these strains are indistinguishable without using molecular identification techniques.

Fall armyworm moths lay their eggs in masses, and, unlike beet armyworms, the larvae quickly disperse from the egg mass. First and second instar larvae are difficult to distinguish from other armyworm species and bollworms. Fall armyworm larvae are usually greenish-brown with a white line below the top of the back, a brownish-black stripe above the midline, and a pale stripe with a reddish-brown tinge below. Their most distinctive characteristic is a prominent, white inverted “Y” on the front of the head. Black hairs on their bodies distinguish them from beet armyworms. Because bollworms also have hairs on their bodies, this characteristic may lead to confusion between small fall armyworms and small bollworms. Fall armyworm larvae also have four large spots that form a square on the upper surface of the last body segment.

When larvae are large, these spots help differentiate fall armyworms from bollworms.

Depending on the strain of fall armyworm, damage to cotton may be nonexistent to severe. Although rare, rice strain larvae have been seen dispersing from pastures into cotton where they fed exclusively on grassy weeds. The corn strain larvae, however, can do extreme damage to cotton. Fall armyworms that feed on cotton are typically more damaging than other armyworm species because they tend to feed on fruiting structures, especially bolls. However, they do not feed as voraciously on fruit as bollworms do and their populations develop more in grain crops such as corn and grain sorghum in preference to cotton.

When abundant in prebloom cotton, fall armyworms may cause defoliation, but the greatest damage comes from topping the plants) branches cut off and stalks almost severed. The most damaging populations of fall armyworms are those that occur during boll filling.

Suggested Insecticides and Rates for Managing Fall Armyworm in Cotton
Insecticide (Trade name)	Lb active ingredient per acre	Amount of formulated per acre	Acres treated per gal or lb of formulated	Mode of Action Group (IRAC)
Transgenic seed
Cry1Ac, Cry2Ab (Bollgard II)	—	—	—	11
Cry 1Ac, Cry1F (Widestrike)	—	—	—	11
Foliar sprays
Chlorantraniliprole1 (Coragen 1.67)	0.045-0.09	3.5-7 fl oz	36.6-18.3	28
Flubendiamde1 (Belt 2 SC)	0.03-0.047	2-3 fl oz	64-42.7	28
Novaluron (Diamond 0.83)	0.058-0.078	9-12 fl oz	14.2-10.7	15
1use a medium to high rate or a low rate tank mixed with a pyrethroid; pyrethroids alone at high rates only offer suppression.